Photoconducting devices



De- 30, 1958 G. s. BRIGGS ET AL 2,866,878

PHOTOCONDUCTING DEVICES Filed April 29. 1955 faz (d5-,450% (die 25% am75% M2 /aa/Z @die INVENTORS lafms'zaas wp BYEAJPH 144' CME/:22mm

irraiA/ir PHOTOCONDUCTHNG DEVICES George S. Briggs and Ralph W.Christensen, Lancaster',

Pa., assignors to Radio Corporation of America, a corporation ofDelaware Application April 29, 1955, Serial No. 504,?11

6 Claims. (Cl. 201--63) This invention relates to improvedphotoconducting compositions which are particularly useful in gap typeand area type photocells and in other devices'utilizing photoconductivebodies. The invention includes improved methods for preparing theimproved photoconducting compositions and improved devices utilizing theimproved photoconducting compositions of the invention. i

A photoconductive device is one which displays a reduced resistance toelectric current flow when irradiated, for example, with light. In itssimplest form, a photoconductive device comprises a photoconductive bodyand a pair of electrodes attached thereto. With a voltage applied to theelectrodes, the device passes an increased amount of electric 'currentwhen there is .an increase in the intensity of light irradiating thedevice.

Ideally, a photoconductive device is a perfect insulator when light towhich it is sensitive is absent, and is a perfect conductor when amaximum intensity of light to which it is sensitive is present.Actually, a photoconductive device behaves as a higher resistanceconductor when light to which it is sensitive is absent, and behaves asa lower resistance conductor when light to which it is, sensitive isincident upon the photoconductive body.

The change in conduction produced by a unit variation of light intensityis referred to as the photosensitivity of the device. The measure ofphotosensitivity is in terms of photocurrent under standard conditions.The current passed by the device in darkness is referred to as the darkcurrent; the current passed when the device is `irradiated is referredto as the light current; and the difference between the light currentand the dark current is referred to asV the photocurrent.

Ice

cells of a single composition exhibit a change in photoconductivity overperiods of time. ln some cases the photoconductivity increases, while inother cases the photoconductivity decreases. It is desirable that suchphotocells maintain substantially constant photoconductivity so thatthey may be incorporated into apparatus without compensating networksand devices.

An object of the inventionl is to provide improved photoconductingcompositions, improved bodies of said photoconducting compositions andimproved photoconducting devices.

Another object is to provide improved photoconducting compositions,bodies and devices that are responsive to a broad band of frequencies inthe electromagnetic spec.-

trum.

Another object is to provideimproved photoconducting compositions,bodies and devices that maintain substantially constant photoconductingproperties with respect to time.

A further object is to provide improved methods for .preparing theimproved photoconducting compositions of the invention.

. .Photoconducting compositions according to the invention comprise anintimate physical mixture of timely-diyided crystal particles of atleast two different photoconducting materials, preferably dispersed inan electricall 1y-insulating, film-forming vehicle. An intimate physicale fl One type of photoconductive device comprises a single crystal of aphotoconductive material and electrodes attached to the crystal. Suchsingle crystal photocells exhibit large photocurrents and high ratios oflight current to dark current. However, the crystals are small in sizeand consequently the total current passed by a single crystal is small.When greater currents are passed through the crystal, the crystal heatsup and the photosensitivity of the crystal is reduced, eithertemporarily or permanently. Furthermore, single photoconductive crystalsare difficult to grow and are fragile. Thus, the expense of manufactureand maintenance often prohibits the use of such single crystalphotocells.

Another type of photoconductive device comprises a body includingfinely-divided photoconducting powder particles of a single compositionand electrodes attached to said body. The body may include, for example,an unbonded powder or a powder mixed with a binder such as a-syntheticresin.' Such powder-type photocells may be made in any desired size,shape` or currentrcarrying capacity. Although such powder-typephotocells often respond` to a 'broader band of frequencies thancrystal, photocells, it is frequently desirable to provide photocellslof this'type that respond to still broader bands of frequencies.

It has been observed that previous powder-ty ev photo-` mixture of atleast two photoconducting materials has been found to provide a broaderspectral response and J better electrical properties than finely-dividedcrystal par- `t'icles of a single photoconducting material.Photo-conducting devices comprising bodies of the improved compositionsof the invention, are effectively more photosensitive due to the broaderspectral response and are more stable with respect to time.

A method of the invention includes intimately mixing finely-dividedcrystal of at least two different photoconducting materials preferablyby dispersing in an electrically-insulating, film-forming vehicle.

The devices of the invention include a body having aYcomposition.according to the invention and at least one electrodeattached to saidbody.

The invention will be more fully described in the following detaileddescription when read in conjunction with the drawings herein:

Figure 1 is a perspective view of a first photocell ac-l cording to theinvention,

Figure 2 is a family of curves illustrating the change inphotosensitivity with: respect to time of compositions according to theinvention and of the component photoconductors of said photocouductingcomposition, and

Figure 3 is a perspective view of a second photocell cording to theinvention.

Similar reference characters are used for similar elements throughoutthe drawings. u

A preferred method for preparing a photoconducting composition accordingto the invention follows.

Fifty 'parts by weight of photoconducting cadmium sulphide and fiftyparts by'weight of photoconducting cadmium selenide are dispersed in ftyparts by weight of a 1% solution of ethyl cellulose in amyl alcohol. Themixture is then coated on a substrate and the amyl alcohol removed byYevaporation.V The composition of the resulting body calculated from'theinitial mixture is 50 parts photoconducting cadmium sulphide, 50 partsphotoconducting cadmium selenide and 0.5 parts ethyl cellulose (allparts by weight).

`(,an'mnfm sulphide photocomuctz'ng powder.-A preferred photoconductingcadmium sulphide powder is prepared as follows: An intimate mixture of100 grams of precipitated cadmium-sulphide, l0 grams of cadmiumchloridefly gramof ammonium chloride, 1 .7millilitersl Qi 0.1.Mcuprecbloride. andv 250 millilitersv of water is, prepared. Thismixture may be prepared in a blender such as is used for mixing powderswith water. The yellow; viscous slurryis dried; at; about 15.0" C. forabout l5 hours.

The dried cakeis then brokenzupdntopea-size lumps and; packed; into a 12inch test tube to. aV depth of about seven;inchestThetubeis providedwith. a. stopper having an inlet tube therethrough.` for the purpose ofmaintaining a substantially stagnant atmosphere in the test tubeWhileamaintainingatmospheric pressure through the subsequent liringsteps; The testY tubevfilled with'. the dried mixture isfired atlaboutk600'. C; for about 2.01 minutes. The fired product is then remo-ved fromthe test tube andallowedi to. soak inwater-until it disintegrates. Thisordinarily takes about 2O` minutes. Thel product is Washedonia fine,sintered, glassfilter, dispersing the cake once or twice in water untilthe washings contain less than-:01 M cadmium chloride.

The product of the first fir-ing is brown in colorand of av relativelyvfine particle, size. During the rst firing, there is present in thecharge about cadmium chloride which is a solvent ux forcadmium-sulphide. The smalll particles of: cadmium sulphide partially orcompletely dissolved in the cadmium chloride and are recrystallized intosmall crystals.- which are o f the order of- 0.3 mill in size and haveycopper andl halide incorporatedv therein. At this stage, theproduct isphotoconductmg.

The washed product of` the first tiring is moistened with aY solutioncontaining equal parts of 0.14 M aqueous cad mium chloride and 1.0y M`ammonium chloride. The excess solution is removed by suction. Afterdrying, the powder is passed through a 325 mesh sieve andthe tailingsdiscarded.

The dry,A sieved powder is placed in a test tube to a depth not greaterthan 4.5 inches. and ired for about minutes at 600 C. in a stagnantatmosphere. The tired mass is removedl from the furnace andpermitted tocool. During this second firing, the powder sinters to a stick, which isthen grated through a mesh sieve. During the second tiring there ispresent only a controlled trace ofsuperticialchloride. The` slightlysinteredV stick is easily broken up into a powder which exhibits a lowdark resistivity. and high. dark current.

Aboutf0.2 gram of sulphur is placed in thek bottom of a test tube andthe sieved brown: powder from the second firing` is placed on, top ofthe sulphur to. a depth of about 4.5 inches. The powder in the test tubeis tired at about 500 C., for about 10 minutes ina stagnant atmosphereand then, while still in the thirdy tiring, a vacuum is applied to thepowder by. means of anV inlet tube and the firing, continued for aboutltl-minutes.V with the vacuum applied. The test tube is removed from thefurnace, cooled and the product passedthrough a 325 mesh. screen..During the third tiring, the sulphur vaporizesV and passes through themass of brownA powder. Thel product of the third firing exhibits ay highdark resistivity, a low dark current anda high. photosensitivity.Typical measurements, indicate the ratio of light current to darkcurrent of about; 10S- and a high speed of" response.

. The cadmium sulphide photoconducting powder which is, the product of;thethirdV firing istan to dark brown in Color,` the color darkening withincreases in either the proportions of copper or increasesof the firsttiring tem# perature. The. average particle size varies according to thefirst tiring temperature, being of the order Yof 0.34 mil for 600 C. andofthe order of 0.7 milA for 650' C. The powders exhibit a panchromaticabsorption, although the spectral response is peaked in red and ispractically nil 'in the blue region of the spectrum. TheV powder isnon-luminescent, has substantially uniform particle size audisfreediowing.

Since. Cadmium. sulphide isu partially. soluble, in4 molten cadmiumchloride, it is believed that the presence of about 10% cadmium chloridein the mix during the rst firing permits the growth of discreteuniformly sized crystals bonded by the Water soluble cadmium chloride.The tired lump disintegrates readily in water. Thus, the content -ofcadmium chloride isnot critical, its purpose being principally toprovide a crystallizing me-dium for the cadmium sulphide host crystal.Although cadmium chloride is preferred, any crystallizing medium for thehost crystal which does not otherwise adversely eiect the product, maybe used in place of cadmium chloride. Similar crystallizing media areused for other host crystals.

Ammonium chloride is introduced into the mix to (1) convert to cadmiumchloride any cadmium oxide which may be present in the mix, and (2) toprovide a tiring atmosphere that prevents oxidation. An activatorproportion of chloride is incorporated into the host crystal during thelirst.` firing.v This. amount is extremely. small and may come fromeither the cadmium chloride or the ammonium chloride.

Copper is introduced into thernix in a proportion equivalent to..100partsper million of copper with respect to. cadmium. sulphide. Theamount of copper is not critical; however, it is preferred to usebetween 50 and 300y parts. per million of copper. In place of copper,other monovalentl cations may be incorporated into the cadmium. sulphidehost crystal. For example, 200 parts permillion; of' silver in placeofcopper produces an orange-colored powder having an intermediatephotosensitivity andi a low rate of decay.

The firing temperature during the first ring is somewhat-critical. Thefiring temperature shouldhbe above the melting point of cadmium chloridewhich is about 580 C. Belowl this temperature practically no crystalgrowthy occurs and the copper does not diffuse into the host crystal.Higher temperatures during the first firing produces a powder which hasa darker color, larger V particle size, lower dark resistivity andhigher dark current. The preferred temperature is the lowesttemperaturerthat insures prompt melting of the solvent material,produces a small particle size and a high dark resistivity inthe finalproduct. A temperature of about 600 C. ispreferred.

The second firing sinters the powder into a stickand increasestheconductivity and photosensitivity of the material. Small particles areprobably sintered onto the surface of the largerones, thus, reducing thenumber of particle-to-particle contacts. Again, aV tiring temperatureofthe order of 600-'C. is, preferred as the lowest ternperat-ure whichinsuresv the prompt melting of cadmium chloride.

During thethird firing, the sulphur vapor which passes through the massof photoconducting powder reduces the darkicurrent of the powder,kpresumably by diminishing the chloride in the powder toa valuesubstantially equivalent to the amount of copper present in` theproduct. At 500 C., the powder does not sinter and the photosensitivity`of the powder isY only slightly affected. At higher temperatures, theloss in` photosensitivity is greater. f A In eachof 'the tiring stepsenough. time should herallowed to b ri1, 1 g the` entire charge tothefurnace temperature. Eortubes about oneinch in` diameter, about 2,0minutes. is, required. Longer tirings up to one hour make no noticeabledifference in the powder. Similarly, the speedY with ,which theproductis` cooled makes little or n.0 difference. in the unal, product.

VGrindirlg theinishelpowder progresively reduces its photosensitivity,similar to,V the observations on single crystals. Grinding of,n an.intermediate product isunde. sirable because it produces all particlesizes and shapes, and. because srindinsis inherently uncontrollable. ltis best, therefore, to avoidV grinding at any stage. The

powder from the first tiring is putthrough a 325 mesh sieve to eliminatethe few lumps which may have formed in earlier steps in the process andalso to establish an upper limit (such as 1.7 mils) to the particlesize. After the second tiring, the 50 mesh sieve is used to achieve amore uniform disintegration of the sintered stick and to avoid crushing.After thethird firing the final product is passed through a 325 .meshsieve to eliminate any aggregates over 1.7 mils. .Less than 5% is lostat this stage. In passingmaterial through a'sieve no hard rubbing isused. Y

The electrical properties of the nal product are influenced by theamount of chloride present during the second firing. With too muchchloride, the final product has a high dark current; with too littlechloride, the final product has a low sensitivity.

Cadmium selenide photoconducting powder.-A preferred photoconductingcadmium selenide is prepared by the general method described for thecadmium sulphide photoconducting powder except that cadmium selenide issubstituted for cadmium sulphide in the starting mixture. Thus, thestarting mixture comprises 100 grams of cadmium selenide, grams ofcadmium chloride, 1 gram of ammonium chloride, 1.7 milliliters of 0.1 Mcopper chloride and 250 milliliters of water.

AReferring to Figure 1, a photocell according to the invention comprisesa pair of spaced electrodes 53 attached to a photoconducting body 55 ofthe invention. The device may be prepared according to the followingprocedure. A glass plate 51 is provided with -a strip of conductingmaterial 200 mils wide and having a 20 `mil gap running across and atright angles to the strip forming two electrodes 53. Such a strip may beproduced by masking the plate 51 and then spraying with a silverresincomposition. Other methods may be used, for example, painting or silkscreening. f l

A mixture is prepared comprising about 100 parts by weight ofphotoconducting powder `and parts by weight of a solution containing oneweight percent of ethyl cellulose resin dissolved in amyl alcohol. Adrop of this mixture is placed on the gap and allowed to dry. When thedrop 55 is dry, the photocell is ready for use. The photoconducting body55 comprises 100 weight parts of'photoconductor and 0.5 Weight parts ofethyl cellulose.

While the photocell of Figure 1 utilizes a photoconducting body 55comprising a resin-bonded powder, an unbonded powder may also be used. Aphotoconducting body bonded with an electrically-insulating,film-forming Vehicle is preferred. It is preferred to use 0 to l weightparts of a binder to 100 weight parts of the photoconducting powder. Inthe above described example, between zero and 100 parts of the 1% ethylcellulose solution may be used with 100 parts of photoconducting powder.Increased amounts of ethyl cellulose increases the resistivity of thephotocells although the above range of constituents is not critical.Other electrically-insulating, film-forming vehicles may be used as abinder, for example, a silicone resin, araldite resin or acryloid resin.In addition to photoconducting Cadmium sulphide and photoconductingcadmium selenide, other photoconductors such las photoconducting cadmiumsulfoselenide and cadmium telluride may be used. The photoconductingcompositions of the invention comprise generally mixtures offinely-divided crystal particles of at least two differentphotoconductors which mixtures may be bonded orunbonded. Thephotoconducting cadmium sulphide and cadmium selenide may be mixed inany proportion, each mixture having its owndistinctive electricalproperties.

Referring to Figure 2, the photocouductivity of the photocells of Figure1 comprising only the preferred photoconducting cadmium sulphidedispersed in ethyl cellulose increase with time as shown by the curve31. The photoconductivity of the photocells of Figure l comprising onlythe preferred. cadmium .selenide dispersed in ethyl cellulose-decreaseswith time as shown by the curve 33. In either case, these photocellschange with time, making it extremely diicult to design the photocellsinto circuits with other components. The stability of these photocellsis improved by utilizing the compositions of the invention, preferably amixture com'- prising 50%A byv weight of photoconducting cadmiumsulphide and 50% by weight of photoconductingycad mium selenidebondedpwithethyl cellulose. As shown by the curve 37,therphotoconductivity of the mixture remains substantially constant withrespect to time. t

There is also shown characteristics of photocells comprising mixtures of25% by Weight photoconducting cadmium sulphide and 75% by weightphotoconducting v cadmium selenide 'bonded with .ethyl cellulose by thecurve'35, and 75 by weightnof photoconducting cadmium sulphide and.2,5%V by weight of photoconducting cadmium selenide bonded withethylljcellulose by the curve 39. In each case there is shown animprovement instability -with respect to time overphotocells made withonly one constituent photoconducting powder.

Table I gives comparative'V data of the operation of typical photocellsconstructed according to Figure l, when irradiated with 0.1lumen*.(73`'footcandles) of light from an incandescent source. Y'I `he'spectral response of photocells comprising only the Apreferredphotoconducting cadmium sulphide powderA bonded with ethyl cellulose ispeaked at about 7500-A. as shown in column 5. The spectral responseofphotocells comprising only the preferred photoconductingcadmiumVselenide powder bonded with ethyl cellulose is'peake'd at'about 9250A.as shown in column 1. As shown in column 3, the spectral response lofphotocells comprising a mixture of equal parts of photoconductingcadmium sulphide and photoconducting cadmium selenide bonded with ethylcellulose is peaked at about 7500 A. and is more sensitive over abroaderrange ofthe spectrum than photocells comprising only one of theconstituent photoconducting materials. Further, a photocell comprisingone of the mixtures of the invention appears to have its own distinctivecharacteristics rather than having characteristics which is the meanl ofthe characteristics o-f the constituents. As shown in columns 2 and 4photocells cornprising other mixtures of the component photoconductingpowders bonded with ethyl cellulose exhibit an increased photoconductiveresponse over a broader range of the spectrum than either componentphotoconductor, and, each mixture exhibits its own distinctive peakspectral response. A similar effect is obtained with photocellscomprising mixtures of photoconducting powders without a binder.

Referring now to Figure 3, another type of photocell is the area. typephotocell which may comprise a subtrate 61, such as porcelain, mica,Bakelite, but preferably glass, upon which has been formed a pair ofinterdigitated electrodes 63. A layer 65 comprising a photoconductingcomposition according to the invention is coated over the gap area ofthe electrodes. A typical device has a gap width of 20 mils and a gaplength of 1000 mils. The electrodes 63 are then connected to a voltagesource 67 external to the device.

The compositions of the invention may be utilized to preparephotoconductive devices and elements that are useful in meters, relays,picture converters, picture intensitiers, pickup devices, switches andso forth. The devices comprise 4a body of a photoconducting compositionof the There has also been describedV4 novel methods for preparing theimproved' photoconducting, compositions of the in vention, andphotoconducting bodies andphotoconducting devices comprising the'improved phtoconducting compositions of the invention. The improvedphotoconductingY compositions of' the invention provide a bro-aderspectral response. and better electricalA properties than finely-dividedcrystal particles of a single photo- .conductingA material;

TABLE'I Relative spectral. response (normalized) .of powder-typephotocells in, arbitrary urritsj Compositions,

CdSE, percent 100 7,5, 50k 25y v 0 CdS, percent A 25 50 75 100 ethylcelluloso.....r.. 0.*5 r 0. 5- 0. 5 0; 5 0. 5

0. 0 0 0 0 0 3. 16 31 5 0 11 91 200 46 0, 45. 430 1,100 460 0 100 7001,'500 800 0 140 1,000. 2,000 1, 400 30 320 1,300 2,7400 2,200 120 5001, 500 2,1300 2, 600 200` 640 z 1,450. 2,000 2, 400 2504 740 1,300l l1,500 1,700 290l 770 1,000 1,000 1, 000 280 680 700 '500 `300 220 500400 100, 60

Irradlatlon= 0.1 lumen from an incandescent source.

Gap=20 x 200 mils.

Applied vo1tage=90 voltsD. C.

What is claimed;

l. A 25 to 75 weight percent photoconductive. body consistingessentially ofk aA physical mixture. of nely- Idivided crystal particlesof photocouducting cadmium sulphide and 25: to 75 weight percentphotoconducting cadmium selenidedispersed in an electrically-insulating,lm-forming vehicle.

2. A photoconductive body consistingessentially of a physical mixture ofiinely-divided crystal particles of photoconducting cadmium` sulphideand photoconducting cadmium selenidedisperseds in a binder, said mixturecomprising about parts by Weight of photoconducting cadmium sulphide7about 50` partsby weight of photoconductin,7 cadmium selenide and about0.5 part by Weight of ethyl. cellulose.

3'. A photoconductive device comprisingv a body ofa physical mixture offinely-divided crystal particles of photoconducting. cadmium sulphideand photoconducting cadmium s-elenide dispersed in anelectricallyainsulating, film-forming, vehicle, said mixture consistingessentially of 50 parts by weight of photoconducting cadmium` sulphide,50 parts vby Weight off photoconducting cadmium 'selenide and` 0.5 partby weight of ethyl cellulose, and at least one electrode in contact withsaid body.

4. A photoconductive body consisting essentially. of a physical mixtureof finely-divided crystal particles olif 25 .to Weight percentphotoconducting cadmium sulphide and 25 to 75I weight percentphotoconducting cadmium Iselenide dispersed in up to, one weight percentethyl cellulose.`

5. A photoconductive body consisting. essentially. of a physical mixtureof finely-divided crystal particles of photoconducting cadmium sulphideand photoconducting cadmium selenidedispersed in` a binder, said mixturecomprising about 25 to 75 parts` by weight photoconducting7 cadmiumsulphide, about 25 to 75 parts by Weight photoconducting cadmiumselenide and about 0.5 part by weight of an electrically-insulating,nlm-forming vehicle;v the total parts cadmium sulphide, and cadmiumlselenide 'be- .ing 100,

6. A photoconductive device comprising a body, con,- sisting essentiallyof ay physical mixture of finelydivided crystal particles of 25; to 75weight percent photoconducting*v cadmium sulphide and 25 to 75 lWeightpercent photoconducting cadmium selenide dispersed` in an.electricallyinsulating, film-forming vehicle, and, at least oneelectrode in contact with said body.

References Cited` in the file ofthis patent UNITED STATES, PATENTS2,552,626 Fisher et al. May. 1,5, 1951 2,647,066 Homer V- V V .f.. Iuly2S, 1953 2,651.700 Gans V A s Sept. 8, 1953 2,710,813, Forgue --.p aJune 14, 1955 2,756,385 Thomsen Oct. 2, 19,56

UNITED STATES PATENT QEETCE CERTIFICATE OF CORRECTION Patent No.@86657878 December BO5 1958 George 5 Briggs et alo It is hereb'rcertified that error appears in the printed specification of the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column '7, line 18, Table I, first column thereof, for "GdSE" read CdSelinev 37, strike out "25 to '75 Weight p'ere'entY; line 39, before"photoeondueting" insert 25 to '75 Weight percent al1-n,

Signed and sealed this: 21st day of July 195% (SEAL) Attest:

KARL Hhl AXLINE Attesting Officer ROBERT C. WATSON Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION December'50, 1958 Patent Not 2oy878 George 5,. Briggs et alf,

It is herebY certified that error appears in the-printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 7, line 18, Table I, first column thereof, for "GdSE" read GdSeline 37, strike out "25 to '75 weight pereentf'; line 39, beforenjplflotooonchioting insert m- 25 to '75 Weight percent a Signed andsealed this' 21st day of July 19591,.

(SEAL) Attest:

KARL Ht AXLINE Attesting OHcer ROBERT C. WATSON Commissioner of Patents

6. A PHOTOCONDUCTIVE DEVICE COMPRISING A BODY CONSISTING ESSENTIALLY OFA PHYSICAL MIXTURE OF FINELY-DIVIDED CRYSTAL PARTICLES OF 25 TO 75WEIGHT PERCENT PHOTOCONDUCTING CADMIUM SULPHIDE AND 25 TO 75 WEIGHTPERCENT PHOTOCONDUCTING CADMIUM SELENIDE DISPERSED IN ANELECTRICALLYINSULATING, FILM-FORMING VEHICLE, AND AT LEAST ONE ELECTRODEIN CONTACT WITH SAID BODY.